1. Field of the Invention
The present invention relates to a method for preparing a dielectric thin film, and more specifically, to a method for preparing a dielectric thin film with a high dielectric constant and a method for preparing a semiconductor device using the same.
2. Discussion of the Related Art
Generally, the area of semiconductor device capacitor is reduced as the degree of integration of the semiconductor device increases. As a result, the thickness of the dielectric film of the capacitor is reduced in order to compensate for the reduction of the condensing capacity of the capacitor. However, as the thickness of the dielectric film is reduced, a leakage current caused by tunneling increases. This leakage current in turn lowers the reliability of the semiconductor device.
To avoid having to reduce the thickness of the dielectric film, one method which increases the effective area of the capacitor by forming very complicated surface curvatures at storage node has been widely used. Although this method enables one to avoid reducing the dielectric film when combined together with a nitride film/oxide film laminating structure or an oxide film/nitride film/oxide film laminating structure having a high dielectric constant, such a method makes the process for photolithography difficult due to the complicated surface curvature. It also leads to a high manufacturing cost. Due to the above problems, it is predicted that such a method will be difficult to use in a highly integrated element (e.g., a DRAM with a storage capacity greater than 256 M).
To greatly enhance the condensing capacity of a capacitor and reduce its surface curvature, a method using substances with a high dielectric constant as a dielectric film of the capacitor has been proposed and many studies have been conducted. For example, Ta.sub.2 O.sub.5 is most widely studied as a substance with a high dielectric constant for a capacitor. Results related to preparing Ta.sub.2 O.sub.5 as a thin film to improve capacitor properties and thus solving the problems related to the high integration have been obtained. Considering the continued demand for high integration, the real dielectric constant for Ta.sub.2 O.sub.5 is not very high. Thus, the range of its usage will probably not be broad.
At present, interests for a Perovskite type oxide, such as a ferroelectric body, have been raised. Such material is intensively studied to be used as a dielectric for semiconductor devices. The Perovskite type oxide materials include Pb(Zr,Ti)O.sub.3 (PZT), (Pb,La)(Zr,Ti)O.sub.3 (PLZT), (Ba,Sr)TiO.sub.3 (BST), BaTiO.sub.3 and SrTiO.sub.3. However, these substances easily react with silicon and silicide used as substrates. In addition, electrodes are often oxidized due to exposure of their surfaces in a strong oxidizing atmosphere during the procedure for forming the Perovskite type oxide thin films. Therefore, research has been conducted to solve these problems (related to the raw material and structure of electrode).
In particular, among these strong dielectric material, BST{Ba,Sr)TiO.sub.3 } shows a paraelectric property at room temperature. It also has a relatively low leakage current property in addition to a high dielectric constant value of more than 2000. Because it is directly applicable to present DRAM designs, BST can be used more easily in a process for preparing the present DRAM capacitor. Thus, much research is concentrated on integration process of semiconductor devices using BST. However, these predominant properties of BST deteriorate during the course of being formed into a thin film. When BST is processed so thin as to use for a device with a high integration (e.g., more than 256 M DRAM), its dielectric constant value is decreased to 100-500, and its leakage current is increased. Therefore, confidential information stored in the device is not maintained and the reliability of the device is lowered.
The leakage current of a BST thin film varies according to the formation process of the thin BST film, the electrode material, or the electrode structure. If a Pt electrode is used instead of a RuO.sub.2 electrode, the BST thin film generally shows a low leakage current because the leakage current is mainly caused by Schottky radiation from the electrode to the dielectric film.
In theory, the work function difference between the electrode material and the dielectric film differs according to the electrode. However, even though a RuO.sub.2 electrode is used, the same level of low leakage current similar to that of the Pt electrode can be obtained, if BST is formed by epitaxial growth. Therefore, such work function difference may not dominantly influences the leakage current.
The results of the above epitaxial BST suggest that the presence of a crystal system increases the leakage current. To reduce such a leakage current, one widely known method is to insert a dielectric layer having a low leakage current together with BST in series between electrodes. For example, one such method using SiO.sub.2, Si.sub.3 N.sub.4 and SrTiO.sub.3 is disclosed in U.S. Pat. No. 4,437,139 and Japanese Patent Laid-Open Hei. 6-350,029.
In this method, BST with the high dielectric constant is combined with SiO.sub.2, Si.sub.3 N.sub.4 and SrTiO.sub.3 with the low leakage current. A substance having the low leakage current is then inserted between the electrode and the dielectric film to reduce the leakage current of the device. Since the thickness of the dielectric film is mostly formed with BST, this method attempts to use only the advantages of the two substances.
So far, BST has been used largely in individual elements of ceramic capacitors. It is usually shaped into fine powders and processed into a sheet form. An electrode is then attached thereto and this is sintered to prepare a capacitor.
Much research aimed at improving various properties of ceramic capacitors using BST has been conducted. In particular, a method where several additives are added in BST to decrease leakage current and enhance sintering property has been proposed. In this method, the dielectric constant is enhanced by increasing semiconductivity. As a result, insulation property is reduced and a substance with high insulation resistance is formed in a crystal structure in order to greatly reduce the leakage current. In U.S. Pat. No. 5,036,424, No. 5,166,759, No. 5,181,157, No. 5,248,640, No. 5,268,006 and No. 5,312,790, compositions which enhance the insulation property are disclosed and additives used are mainly Cu, Mn, Si, Al, Zn, Li, Mg, Fe, Cr, Co, Ni, B, and Pb etc. These substances play a role in insulating the crystal structure present as an oxide form having a high insulation resistance in the crystal structure in the sintering process.
In addition to methods wherein additives are mixed to raw materials, another method, wherein the previously formed BST is heated with Cu at a high temperature and Cu is added into BST by diffusing vaporized Cu into the dielectric, is disclosed in U.S. Pat. No. 4,739,544.
In Japanese Patent Laid-Open Hei 6-350,100, a method is disclosed to enhance the leakage current property of a thin dielectric film. Depending on the conductive property of the dielectric body, a donor is ion injected into the dielectric in the case of a p-type conductivity and an acceptor is ion injected into the dielectric in the case of an n-type conductivity. As a result, the number of charge carriers are decreased by counter-doping to reduce the leakage current.
The conventional methods for preparing a dielectric thin film as described above have the following problems.
First, although the dielectric constant and leakage current of a BST thin film change according to the thin film formation process, the dielectric constant and leakage current are generally in complementary relationship with each other. In other words, the leakage current is generally high in case of a high dielectric constant and the leakage current is low in case of a low dielectric constant. As a result, efforts to reduce the leakage current by controlling the formation condition of the BST thin film are generally accompanied by a reduction of the dielectric constant.
Accordingly, when a substance with a high dielectric constant (e.g., BST) is used in combination with a substance with a low leakage current, the final leakage current can be reduced. However, since substances with low leakage currents are generally materials with low dielectric constants, it is inevitable that the total dielectric constant is lowered when such material is used in combination with BST.
A related problem occurs when combining substance with a low leakage current with BST. In the operation of DRAM, both directional and anti-directional voltages are usually applied to capacitors. Thus, symmetry in the structure itself should be maintained since the same properties hold with respect to the applied voltages in both directions. As a leakage current prevention film is inserted in each interface of BST and both electrodes to maintain the symmetry, the proportion of substance with a low dielectric constant becomes high and the total dielectric constant will be further reduced.
The second problem associated with the conventional methods is that they are only suitable for certain processes. In a method for preparing a ceramic capacitor individual element, the most widely used method for preventing the leakage current is to use additives to insulate the crystal system. More specifically, before the dielectric shaping, additives are mixed together with the BST raw material powder. After the shaping, electrodes are attached thereto and the resulting product is then sintered to prepare a capacitor. Such a method is not suitable for a process for preparing an individual element with a bulky volume, nor suitable for a direct circuit process using fine elements.
Third, although the conventional process (wherein the BST dielectric is shaped and vapor-phase Cu is then diffused in BST) may be used to prepare integrated circuits, the semiconductor base plate has to be charged in a container maintained with a Cu atmosphere and annealed. Accordingly, a closed environment is required to maintain the metal vapor (Cu) atmosphere and to insulate the harmful ingredients contained in the metal vapor from human operators. In addition, when a substance having a low vapor pressure is used, it is heated to exceeding high temperatures to form additive atmosphere. Furthermore, because these metals are easily oxidized materials, oxygen has to be maintained at a very low concentration during the diffusion of the additives.
Fourth, the method of adding impurities by ion injection has the following problems. This method decreases conductivity by reducing the concentration of charge return particles in a crystal structure of dielectric. Hence, if the crystal system is a main path of the leakage current, this method is not very effective. In addition, production cost becomes high because ion injection is a expensive process.